Where bodies for method of this interfeace are defined?
You'll see plenty of others too. Because the
Servlet Specification tells container providers what
they have to implement.
They leave container providers write concrete classes, You can imagine that
Tomcat's implementation will be different from Weblogic's. But they will do the same job, as written in the specification.
When you are written your own servlets, you don't care what the concrete class is. You work on interfaces. That's why your servlets will work on any container.